The present invention relates to an electro-optic device comprising a transparent conductive layer, an electrochromic layer applied on the said transparent layer, these two first elements constituting together a first electrode, a counter-electrode maintained at distance from the said first electrode, and an electrolyte ensuring an ionic binding between the electrode and the counter-electrode.
It is understood, by "electrolyte", any conductive medium ensuring ionic connection, this connection being of cathodic and/or anionic type.
It is understood, by "electrochromic layer", a layer made of a material the characteristic of absorption of the electromagnetic radiations of which is modified, even at room temperature, under the influence of an electric field. Such material, for instance, can show a weak absorption in the visible spectrum in the absence of an electric field and then be almost transparent; however, when they are submitted to an electric field, they absorb for instance the red end of the spectrum, so that they exhibit a blue color. Similar effects can be found in other portions of the spectrum of the electromagnetic radiations, as well invisible as visible.
When an electrochromic layer is deposited on a conductive transparent substrate, one can vary the optical density of this layer while applying an electric field between the electrodes thus constituted and an auxiliary electrode, or counter-electrode, the space comprised between the two electrodes being filled with an electrolyte, the whole constituting a kind of "sandwich". The application of a difference of potential to the terminals of the two electrodes permits to bring the electrochromic layer from the "clear" state or uncolored state to the dark or colored state.
One knows already such devices. Especially, it is known to deposit the electrochromic layer so that it constitutes a pattern. Several of these patterns can constitute together a data. One knows, for instance, an electro-optic display device comprising seven segments, which permits to represent, by a suitable selection of these segments, all the arabic data from 0 to 9. When the segments are alternatively colored and uncolored, the data appear and disappear successively; one calls "cycle" the succession of a coloration and of an uncoloration.
In the known electrochromic devices, for instance in those which are disclosed in the U.S. Pat. Nos. 3,704,057, 3,708,220, 3,819,252, 3,827,784 and 3,879,108, the electrolyte which is used is generally constituted by a mixture of glycerin and of sulfuric acid. This electrolyte has the drawback that the electrochromic layer disappears progressively, by erosion or dissolution, after a number of cycles or after a storage time. The process of dissolution or of erosion is presently not very well known but the presence of water and of complexing agents could be the origin of this phenomenon.
The use, for the realization of the electrolyte, of aprotic solvents (which do not contain protons) in which is dissolved, for instance, lithium perchlorate, gives rise to technologic difficulties: The drying of the electrolyte so that it does not contain water, the rate of the process of coloration-uncoloration which is slower than with a protonic electrolyte since one injects a cation larger than a proton, and the sealing of the electrochromic cell, for instance. If an ionic conductive electrolyte, such as mentioned in U.S. Pat. No. 3,712,710 and which is solid at room temperature is used, the problem of the contact at the electrode-solid electrolyte interface must be solved so that the electric contact will not be lost during the operation.
The purpose of the present invention is to remove these drawbacks.